As is known, wafers for the semiconductor industry have been bonded together by a bonding layer in a sandwich type relation for various applications. Further, for these applications, the top wafer diameter needs to be reduced while the diameter of the bottom or handling wafer remains unchanged. In cases where the bonding layer does not extend to the outer diameter, the top wafer is thinned down to microns of thickness and the unsupported area beyond the bonding layer becomes problematic. The thinning process tends to chip the unsupported wafer edge and the debris created causes scratching and subsurface damage on the prime surface of the wafer. Furthermore, in the chase of wafer thinning processes that utilize wet etching, exposed areas of the bonding layer may be chemically etched or attacked by the wet etching chemicals, thereby compromising the structural integrity of a device containing the top wafer and possibly lead to a loss of yield. A more extreme example of the damage is that large pieces of the top wafer break off with the bonding layer attached.
Prior art methods have been used to try to solve the problem stated above. One such method includes photolithography and etch techniques to remove a perimeter of the top wafer. However, this technique is costly and adds long cycle times to the processing. Another solution that has been introduces to solve this problem is to reduce the diameter of the top wafer by using a fixed abrasive grind wheel on a conventional edge grinder. Such applications are currently performed by companies producing SOI wafers (silicon on insulator) and result in a ledge being formed along the periphery of the handling wafer while the diameter of the upper wafer is reduced. However, the side of the top wafer is not protected and mechanical damage can occur during subsequent planarization and thinning of the top wafer and the bonding layer may be damaged. In the case of wet etching processes, a chemical attack on the bonding layer can create mechanical instability of the device and increase the probability of damage. Furthermore, trapped chemical species on the bonded side of the top wafer can diffuse to other areas of the top wafer and degrade or destroy the device.
Therefore, new wafer thinning methods are needed that prevent chipping of the edges of the top wafer and mask wet etching chemicals from the bonding film during a wafer thinning process.